Metallic liquid normally enters an extremely unstable state when cooled below a melting point, and is immediately crystallized to become crystallized metal. In this event, time for which a supercooled liquid can exist in an uncrystallized state where atoms are randomly arranged, i.e., a so-called “amorphous state,” is estimated to be 10−5 seconds or less at a nose temperature of a continuous cooling transformation (CCT) curve. Specifically, this means that it is impossible to obtain amorphous alloys unless a cooling rate of 106 K/s or more is achieved.
However, there has recently been invented metallic glass which undergoes clear glass transition and is not crystallized even at a cooling rate of 100 K/s or less since a supercooled liquid state is extremely stabilized in a specific alloy group including a zirconium base (see, for example, The June 2002 edition of Kinou Zairyou (Functional Materials), Vol. 22, No. 6, p.p.5-9; Non-patent Document 1).
Each of these metallic glasses has a wide temperature range (supercooled liquid temperature range) in which a supercooled liquid state can be maintained. For this reason, superplastic forming by means of viscous flow can be performed on each of the metallic glasses (see, for example, The July 2002 edition of Kinou Zairyou (Functional Materials), Vol. 22, No. 7, p.p. 5-8; Non-patent Document 2) under a condition that a temperature and a time period are not reached within the temperature range causing crystallization.
Additionally, it is known that a large-shaped amorphous alloy (a bulk metallic glass) can be manufactured directly from molten metal by any one of manufacturing methods such as a water quenching method, an arc melting method, a permanent mold casting method, a high-pressure injection molding method, a suction casting method, a mold-clamp casting method and a rotating-disc fiber manufacturing method (see, for example, The June 2002 edition of Kinou Zairyou (Functional Materials), Vol. 22, No. 6, p.p.26-31; Non-patent Document 3).
Metallic glasses manufactured by these manufacturing methods can provide mechanical properties even in large sizes, which may otherwise be lacking in crystalline alloys. The mechanical properties include a high strength and a low Young's modulus and a high elastic limit, which are inherent in the amorphous state. For this reason, the metallic glasses are expected to be widely put into practical use as structural materials.
Metallic glasses are originally suitable for application to thin-wall molded articles, such as an electronic equipment cabinet, for which three-dimensional shapes realizing high strengths and light weights are favored. There are, however, problems as described below with the above described manufacturing methods of obtaining large-shaped metallic glass components.
Firstly, the permanent mold casting method has the following problems. The general permanent mold casting method is a simple method with which molten metal is simply poured into a molding cavity of a die. Therefore, depending on the shape of the component, it is difficult to avoid shape losses due to insufficient run of spreading of the molten metal, and casting defects such as cold shut and blowholes. Additionally, a cooling rate from the die is unstable, and thus it frequently occurs that part of molten metal is not turned amorphous.
Secondly, the high-pressure injection molding method has the following problems. The general high-pressure injection molding method (for example, Japanese Patent Publication No. Hei 10-296424) is capable of molding a subject into a three-dimensional shape by supplementing an insufficient run of spreading of the molten metal by high-pressure injection. However, a formation of a complicated runner as shown in FIGS. 6 to 8 in Japanese Patent Publication No. Hei10-296424 is required in order to obtain a more complicated shape where a boss, a rib and the like are further provided.
Furthermore, in order to reduce the casting defects as described above, there remains a complication where devices such as an air vent (a gas exhausting passage) and an overflow (a waste molten metal tank) have to be elaborately added.
Defective rates due to the casting defects of the die casting are generally assumed to be several percent to several tens of a percent even by using such techniques based on experiences of those who skilled in the art. This indicates that there is no technique by which casting defects can be innovatively prevented in the high-pressure injection molding method.
Thirdly, a melt-forging method has the following problem. In the melt-forging method or the mold-clamp casting method where a molten metal of a metallic glass, which has been arc melted on a water-cooled copper casting mold, then immediately forged and molded. The copper casting mold is water-cooled from a backside so as to prevent a surface of the mold from being heated to a high temperature and being melted at the time of arc-melting.
In locations of a water-cooled portion which make contact with the surface of the mold, melting is insufficient and the metallic glass is not formed. For this reason, locations not suitable for a finished article remain in the molded article, and there is a disadvantage that these parts have to be removed.
In order to avoid this problem, a forging method has been proposed (refer to Japanese Patent Translation 2003-534925). In the method, the mold and a material alloy are heated together to a temperature equal to or more than a melting point of the metallic glass, and then high-speed molding is performed on the material alloy by pressurization, by use of a mold made of silicon
Nevertheless, although this forging method is applicable to a simple shape such as a plate material, a cutting process of the mold becomes a problem in applying this method to an article having a complicated three-dimensional shape.
Furthermore, in the melt-forging method, since molding is performed by closing the mold at an instantaneous speed, it is difficult to control a thickness of a molded article with high accuracy in the order of 1 mm or less. Accordingly, there is a critical problem that the method is not easily applicable to a thin-wall or nonuniform-wall molded articles.
Fourthly, a press forming method has the following problem. For example, in Japanese Patent Publication No. Hei10-216920, shown is a method of forming a block-shaped amorphous alloy, which has been heated to a supercooled liquid temperature range, by pressing it against an occluded section of a die placed in a vacuum chamber.
In this method, it is extremely difficult to finish the amorphous alloy into a complicated three-dimensional shape where a boss, a rib, a window frame, a hole and like are provided, in a single time of press forming. Furthermore, since arrangement and removal of a heater and a cooling device are repeated, it is difficult to successively form complicated shapes requiring high measurement accuracy in short cycle times.
Consequently, in order to solve the above described problems, the present inventors advanced experiments and researches by trying various methods. They took into account a point that it is only necessary to mainly manage measurement changes due to thermal expansion and shrinkage because solidification shrinkage does not occur when a metallic glass solidifies as supercooled liquid without crystallizing from the molten metal. Accordingly, they obtained a finding that surface defects can be cleared away in the following manner. Firstly, necessary outline measurements and three-dimensional shape sections are formed by performing pre-forming by die casting in which the injection is performed at a high pressure. Then, warm pressing dies forming a cavity conforming to the outline measurements are prepared. Subsequently, a pre-formed semi-article is arranged between the dies heated to a supercooled liquid temperature range, and warm press forming is performed thereon by pressing with the dies. By this way, material surrounding surface defects remained on a surface of the pre-formed semi-article is filled into the surface defects by means of viscous flow.
In addition, they obtained the following finding. A cavity portion is formed in the warm pressing dies in a manner where the cavity portion has a gap of 1 mm or less. Accordingly, finishing forming in which viscous flow specific to the metallic glass is utilized becomes possible, and this is also suitable for a complicated shape having a nonuniform-wall or a thin-wall in three-dimension.
As a result of further continuing ardent studies based on the findings as described above, the present inventors and others reached completion of the present invention.